Resistance spot welding is a process used by a number of industries to join together two or more metal workpieces. The automotive industry, for example, often uses resistance spot welding to join together pre-fabricated metal workpieces during the manufacture of a vehicle door, hood, trunk lid, or lift gate, among others. A number of spot welds are typically formed along a peripheral edge of the metal workpieces or some other bonding region to ensure the part is structurally sound. While spot welding has typically been practiced to join together certain similarly-composed metal workpieces—such as steel-to-steel and aluminum alloy-to-aluminum alloy—the desire to incorporate lighter weight materials into a vehicle body structure has generated interest in joining steel workpieces to aluminum or aluminum alloy (hereafter collectively referred to as “aluminum” for brevity) workpieces by resistance spot welding.
Resistance spot welding, in general, relies on the resistance to the flow of an electrical current through contacting metal workpieces and across their faying interface to generate heat. To carry out such a welding process, a pair of opposed spot welding electrodes are typically clamped at diametrically aligned spots on opposite sides of the workpieces at a predetermined weld site. An electrical current is then passed through the metal workpieces from one electrode to the other. Resistance to the flow of this electrical current generates heat within the metal workpieces and at their faying interface. When the metal workpieces being welded are a steel workpiece and an aluminum workpiece, the heat generated at the faying interface initiates a molten weld pool in the aluminum alloy workpiece. This molten aluminum alloy weld pool wets the adjacent surface of the steel workpiece and, upon stoppage of the current flow, solidifies into a weld joint. After the spot welding process has been completed, the welding electrodes are retracted from their respective workpiece surfaces, and the spot welding process is repeated at another weld site.
Spot welding a steel workpiece to an aluminum workpiece presents some challenges. These two types of metals have several considerable dissimilarities that tend to disrupt the welding process. For one, steel has a relatively high melting point (˜1500° C.) and a relatively high resistivity, while aluminum has a relatively low melting point (˜600° C.) and a relatively low resistivity. As a result of these physical differences, aluminum melts more quickly and at a much lower temperature than steel during current flow. Aluminum also cools down more quickly than steel after current flow has been terminated. Thus, immediately after the welding current stops, a situation occurs where heat is sustained in the steel workpiece and ultimately conducted through the aluminum workpiece towards the electrode on the aluminum workpiece side. A sustained elevated temperature in the steel workpiece and the development of steep thermal gradients between the two workpieces is conducive to the growth of brittle Fe—Al intermetallic compounds at the faying interface. Fe—Al intermetallic compounds, in turn, can reduce the strength and ductility of the weld joint if their growth is excessive.
Another notable dissimilarity between the two metals is that aluminum contains one or more refractory oxide layers (hereafter collectively referred to as a single oxide layer for brevity) on its surface that are created during mill operations (e.g., annealing, solution treatment, casting, etc.) and environmental exposure. This oxide layer, which is composed primarily of aluminum oxides, is electrically insulating, mechanically tough, and self-healing in air. Such characteristics are not conducive to the mechanics of spot welding a steel workpiece to an aluminum workpiece. Specifically, the surface oxide layer raises the electrical contact resistance of an aluminum workpiece—namely, at its faying surface and electrode/workpiece contact interface—thus causing excessive heat to be generated at those interfaces making it difficult to effectively concentrate heat within the aluminum workpiece. The mechanical toughness of the surface oxide layer also inhibits wetting of the steel workpiece. Moreover, the problems posed by the refractory oxide layer on the surface of the aluminum workpiece are further complicated by the fact that the oxide layer can self-heal or regenerate if breached in the presence of oxygen.